The proposed resarch will center on the function of the visual nuclei of the turtle forebrain. The reptilian brain is of primary importance in the study of the evolution of brain mechanisms because the now-extinct stem reptiles were ancestral to both birds and mammals, as well as to the present-day reptiles. Paleontological evidence supports the notion that turtles are the living reptilian group least changed from the stem reptile and thus serve as the best group to study in order to learn about the common ancestors of birds and mammals. The turtle to be studied will be the Eastern painted turtle, Chrysemys picta picta, a species for which a great deal of behavioral data has been collected. We have recently completed an atlas of the brain of this turtle. The proposed research will examine the effects of lesions in the forebrain visual nuclei on the ability of turtles to solve brightness or pattern discriminations. To understand further the function of these areas, subsequent psychophysical experiments will determine the difference thresholds for intensity before and after lesion of one of the visual nuclei. In order to map the location, extent, and connections of these visual nuclei, experimental neuroanatomical studies will be performed prior to beginning the behavioral studies. This research will make it possible to compare reptilian, avian, and mammalian visual functions and will shed light on the evolution of visual mechanisms.